Just be good for goodness' sake? What the blink for?
As a kid, I loved watching the He-Man and the Masters of the Universe cartoon. One time, He-Man and the good guys enlisted the aid of the evil Skeletor to save Eternia from a giant meteor(?). Skeletor agreed reluctantly. In the middle of bad guy/good guy banter, Skeletor asked, "haven't you wanted to do something evil?" to which He-Man replied, "haven't you wanted to do something good?" The world of cartoons of my childhood almost always represented good and evil as a great universal dualism where the good people were the happy ones that always triumphed over evil, the winners in conflict (probably to avoid un-PC theism). And who doesn't want to be on the side of winners? That's how the DC bus stop slogan "Why believe in a god? Just be good for goodness' sake" comes across to me. Good--an end unto itself. Simplistic, child-like, a morality without reason, without ultimate justification, just because.
Yes, well, I'm not a kid anymore, and neither are the atheists trying to campaign anti-God. Let's put things into perspective with a few parallel altruisms:
Why believe in a god? Just be moral for morality's sake.
Why believe in a god? Just be fair for fairness' sake.
Why believe in a god? Just abstain from sex before marriage for abstinence's sake.
Ah, I think the last one casts the issue in the proper light. If you think about it like the others, just being good for goodness' sake just sounds dumb. In the real world, we start asking "why?" at about age four and don't expect 'just because' as a viable answer. We look for a reason to act a specific way, because motivation based on 'just because' isn't motivation enough. Just ask a four year-old.
There are different standards and varieties of "goodness," are there not? Whose goodness shall we emulate? What are the limits to goodness? What are the exceptions, if any? The very heart of the problem is this: Is there such a thing as moral 'goodness' anyway? If you're an atheist, not really. For an atheist, goodness is social convention driven by pragmatic intents of, well, whoever has the most power, whether it be one person or a whole party of persons. OR, the atheist derives goodness from what benefits his own circumstances in life, which may or may not benefit anyone else. The slogan falls into a relativist trap. What is good for you may not be good for me. In any case, whatever 'good' he may appeal to is, on the whole, completely illusory. Moral 'goodness' is artificial, made up. Say, for the atheist, that would make goodness kind of like God--nonexistent!
The bottom line is, goodness is made possible by the existence of God. For the Christian theist, God doesn't simply behave 'good' but personifies goodness as its very source. To answer the ancient question (Euthyphro dilemma), "Is what is good/moral commanded by God because it is moral, or is it good/moral because it is commanded by God," God is good, and He cannot be otherwise and still be God. Goodness subsists in the mind of God and does not exist independently of Him. So an atheist trying to be good just for goodness' sake still pays homage to the fact that there is such a thing as goodness, which entails such a person as God.
We have gotten more sophisticated than hero cartoons from the 80's. Without God, there is little qualitative difference between goodness and evil. Every motivation degenerates into pragmatism, and even atheists can agree that people have done things for pragmatic reasons that are far from good.
So I say, why believe in atheists? Just be logical for logic's sake.
For a lengthier read concerning the Euthyphro dilemma, check out "An Analysis of the Very Complicated Doctrine of Divine Simplicity: Part. 3 of 3" by Jules Grisham.